To permit a person at a periphery to notice robbery, to permit an intruder to stop robbery and to prevent robbery by detecting the opening/closing of the door of a vehicle, measuring time after the door is opened and outputting a signal for reporting robbery when time exceeds reference time.

When a person unlocks the door 6 or breaks window glass and unlocks the door, opens the door 6 and intrudes into the vehicle for the purpose of the robbery of the vehicle and valuables in the vehicle, a timer 3 counts time after the door 6 is opened. When time reaches time b-seconds, which is set in a reference value setting part 4, a start signal is sent to the control part of a born 8 and a hazard lamp 7 form an instruction part. Then, the horn 8 sounds and the hazard lamp 7 flickers. When a possessor opens the door 6, micro warning sound is rung with the start of the timer 3 and a robbery prevention device 1 is released by turning a key to the position of an accessory before time b-seconds, which is previously set.